# MuIndex::type-mismatch


**Key:** `MuIndex::type-mismatch`

**Category:** indexing · **Severity:** error

**Source:** [`src/diag/hints.nix:886`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L886)

the index passed to `con` must have the description's index type

## Example

`Hint::MuIndex::type-mismatch` means the `MuIndex` position supplies a term whose type does not match the expected type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: the constructor index has the wrong type.
con wrongIndex payload

# Better: construct at an index from the declared index type.
con expectedIndex payload

```


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